1. Field Of The Invention
This invention relates to the field of diffraction gratings and optical filters and, more particularly, to rigid three dimensional diffraction gratings and optical filters used as laser protection devices.
2. Description Of Related Art
Plane diffraction gratings are commonly used as narrow bandpass filters or as rejection filters. As a narrow bandpass filter, the diffraction grating disperses the light into its component wavelengths. A suitable slit or aperture permits light within a narrow wavelength band to pass through the grating and to reject, light outside that band. As a rejection filter, a stop placed at an appropriate position in the output of the diffraction grating blocks light having a defined wavelength from passing therethrough and permits light outside that wavelength to pass through. As a rejection filter, the diffraction grating can be used as a protective screen against laser light of a known frequency. More generally, however, a diffraction grating serves as a protective screen from laser light when it functions as a narrow bandpass filter.
A much narrower bandpass filter, which can provide a much greater level of rejection, can be obtained by utilizing Bragg diffraction in a three dimensional grating, equivalent to Bragg diffraction in a crystal. Such a pseudo-crystal consists of a three-dimensional array of equally spaced scattering centers. The Bragg equation governing diffraction in a three-dimensional medium is:
n.lambda.=2d sin .theta., where
n is a positive or negative integer indicating the diffraction order;
.lambda. is the wavelength of the incident light;
.theta. is the angle of incidence of the radiation relative to the direction of the incident beam; and
d is the distance between scattering centers, equivalent to the interatomic spacing in a crystal.
By suitably choosing the values of d and .theta., the filter can be tuned to any desired wavelength, .lambda.. In this way, a narrow band filter can be made to function as either a bandpass filter or as a rejection filter at a preset wavelength. For a fixed value of d, the wavelength can be changed by changing .theta., i.e. by rotating the three-dimensional grating to change the angle of incidence of the grating with respect to the incident beam.
Aqueous-based three dimensional gratings have been developed which provide much higher rejection and much higher resolution than the rigid one dimensional gratings. Such three dimensional aqueous diffraction gratings are disclosed in S. A. Asher and P. L. Flaugh, "Crystalline Colloidal Bragg Diffraction Devices: The Basis Of A New Generation Of Raman Instrumentation", Spectroscopy, Vol. 1, No. 12, pp. 26-31 (1986) and by Asher in U.S. Pat. Nos. 4,627,689 and 4,632,517. These gratings consist of a colloidal dispersion of charged polystyrene spheres in a liquid. An electrostatic charge on each of the spheres produces mutual repulsion, causing the spheres to arrange themselves into a lattice of equally spaced scattering centers. The inter-scatter distance can be changed by adjusting the number density of spheres in the liquid so as to permit Bragg diffraction in the visible region of the spectrum. The array of the polystyrene spheres in water forms a hexagonal closed packed crystal structure.
By using an aqueous-based three dimensional grating, a narrow band filter with a bandpass about 40.ANG. wide and an out-of-band rejection ratio about 99.99% has been produced. Such an aqueous-based grating is usually held between transparent plates of glass or quartz. Because the three dimensional grating is aqueous based, the use of a container for the grating is an absolute requirement. In addition to such awkwardness in use, the aqueous based three dimensional gratings are also limited by their inability to conform to irregularly shaped surfaces, are subject to mechanical instabilities in the presence of vibrations, and are awkward to use and to handle.
The three-dimensional pseudo-crystal suffers in that the grating is in the form of a liquid, and is therefore subject to instabilities produced by vibration, turbulence, thermal gradients, and the like. For ease of application and use, it is desirable that the three dimensional diffraction grating be rigid. A rigid grating can be used with most any surface and in most any orientation. Until now, such rigid diffraction gratings have been one dimensional, consisting of etched lines on a coated medium. Such one dimensional gratings provide only limited resolution and rejection properties. Thus, there is a need for a diffraction grating that combines the ease of use of one dimensional rigid gratings yet obtains the rejection and resolution performance of a three dimensional grating.